Network Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 7234 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Fastly
Expires: September 6, 2018 J. Reschke, Ed.
greenbytes
March 5, 2018
Hypertext Transfer Protocol (HTTP): Cachingdraft-fielding-httpbis-http-cache-00
Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response
messages.
This document obsoletes RFC 7234.
Editorial Note
This note is to be removed before publishing as an RFC.
_This is a temporary document for the purpose of planning the
revisions of RFCs 7230 to 7235. This is not yet an official work
item of the HTTP Working Group._
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
Errata for RFC 7234 have been collected at <https://www.rfc-editor.org/errata_search.php?rfc=7234>, and an additional issues list
lives at <https://github.com/httpwg/http11bis/issues>.
The changes in this draft are summarized in Appendix D.1.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Draft HTTP Caching March 2018D.1. Since RFC 7234 . . . . . . . . . . . . . . . . . . . . . 40
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 421. Introduction
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing
response messages.
An HTTP cache is a local store of response messages and the subsystem
that controls storage, retrieval, and deletion of messages in it. A
cache stores cacheable responses in order to reduce the response time
and network bandwidth consumption on future, equivalent requests.
Any client or server MAY employ a cache, though a cache cannot be
used by a server that is acting as a tunnel.
A shared cache is a cache that stores responses to be reused by more
than one user; shared caches are usually (but not always) deployed as
a part of an intermediary. A private cache, in contrast, is
dedicated to a single user; often, they are deployed as a component
of a user agent.
The goal of caching in HTTP/1.1 is to significantly improve
performance by reusing a prior response message to satisfy a current
request. A stored response is considered "fresh", as defined in
Section 4.2, if the response can be reused without "validation"
(checking with the origin server to see if the cached response
remains valid for this request). A fresh response can therefore
reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it
can be freshened by validation (Section 4.3) or if the origin is
unavailable (Section 4.2.4).
This specification obsoletes RFC 7234, with the changes being
summarized in Appendix A.
1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [MESSGNG].
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This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of
[MESSGNG], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation.
1.2.1. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing
time in seconds.
delta-seconds = 1*DIGIT
A recipient parsing a delta-seconds value and converting it to binary
form ought to use an arithmetic type of at least 31 bits of non-
negative integer range. If a cache receives a delta-seconds value
greater than the greatest integer it can represent, or if any of its
subsequent calculations overflows, the cache MUST consider the value
to be either 2147483648 (2^31) or the greatest positive integer it
can conveniently represent.
Note: The value 2147483648 is here for historical reasons,
effectively represents infinity (over 68 years), and does not need
to be stored in binary form; an implementation could produce it as
a canned string if any overflow occurs, even if the calculations
are performed with an arithmetic type incapable of directly
representing that number. What matters here is that an overflow
be detected and not treated as a negative value in later
calculations.
2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers
([SEMNTCS]) while eliminating the transfer of information already
held in the cache. Although caching is an entirely OPTIONAL feature
of HTTP, it can be assumed that reusing a cached response is
desirable and that such reuse is the default behavior when no
requirement or local configuration prevents it. Therefore, HTTP
cache requirements are focused on preventing a cache from either
storing a non-reusable response or reusing a stored response
inappropriately, rather than mandating that caches always store and
reuse particular responses.
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Each cache entry consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request
target (Section 4.3.1 of [SEMNTCS]). However, it is also possible to
cache permanent redirects, negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and responses to
methods other than GET if the method's definition allows such caching
and defines something suitable for use as a cache key.
The primary cache key consists of the request method and target URI.
However, since HTTP caches in common use today are typically limited
to caching responses to GET, many caches simply decline other methods
and use only the URI as the primary cache key.
If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting
header fields (Section 4.1).
3. Storing Responses in Caches
A cache MUST NOT store a response to any request, unless:
o The request method is understood by the cache and defined as being
cacheable, and
o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 5.2) does not appear
in request or response header fields, and
o the "private" response directive (see Section 5.2.2.6) does not
appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.2 of [AUTHFRM]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and
o the response either:
* contains an Expires header field (see Section 5.3), or
* contains a max-age response directive (see Section 5.2.2.8), or
* contains a s-maxage response directive (see Section 5.2.2.9)
and the cache is shared, or
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* contains a Cache Control Extension (see Section 5.2.3) that
allows it to be cached, or
* has a status code that is defined as cacheable by default (see
Section 4.2.2), or
* contains a public response directive (see Section 5.2.2.5).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 5.2.3.
In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements all specified
caching-related behavior.
Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses.
3.1. Storing Incomplete Responses
A response message is considered complete when all of the octets
indicated by the message framing ([MESSGNG]) are received prior to
the connection being closed. If the request method is GET, the
response status code is 200 (OK), and the entire response header
section has been received, a cache MAY store an incomplete response
message body if the cache entry is recorded as incomplete. Likewise,
a 206 (Partial Content) response MAY be stored as if it were an
incomplete 200 (OK) cache entry. However, a cache MUST NOT store
incomplete or partial-content responses if it does not support the
Range and Content-Range header fields or if it does not understand
the range units used in those fields.
A cache MAY complete a stored incomplete response by making a
subsequent range request ([RANGERQ]) and combining the successful
response with the stored entry, as defined in Section 3.3. A cache
MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response. A
cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code.
3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.2 of [AUTHFRM]) to satisfy any
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subsequent request unless a cache directive that allows such
responses to be stored is present in the response.
In this specification, the following Cache-Control response
directives (Section 5.2.2) have such an effect: must-revalidate,
public, and s-maxage.
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale
(Section 4.2.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin
server.
3.3. Combining Partial Content
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more
Range specifiers ([RANGERQ]). After several such transfers, a cache
might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client
requirements in Section 4.3 of [RANGERQ].
When combining the new response with one or more stored responses, a
cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored
response, unless:
o The presented effective request URI (Section 5.5 of [MESSGNG]) and
that of the stored response match, and
o the request method associated with the stored response allows it
to be used for the presented request, and
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o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma
(Section 5.4), nor the no-cache cache directive (Section 5.2.1),
unless the stored response is successfully validated
(Section 4.3), and
o the stored response does not contain the no-cache cache directive
(Section 5.2.2.2), unless it is successfully validated
(Section 4.3), and
o the stored response is either:
* fresh (see Section 4.2), or
* allowed to be served stale (see Section 4.2.4), or
* successfully validated (see Section 4.3).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 5.2.3.
When a stored response is used to satisfy a request without
validation, a cache MUST generate an Age header field (Section 5.1),
replacing any present in the response with a value equal to the
stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of [SEMNTCS]) to the origin server; i.e., a cache is
not allowed to generate a reply to such a request before having
forwarded the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4.
When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It
can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use.
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When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [SEMNTCS]),
it MUST NOT use that response unless all of the selecting header
fields nominated by the Vary header field match in both the original
request (i.e., that associated with the stored response), and the
presented request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [MESSGNG])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is
also absent there.
A Vary header field-value of "*" always fails to match.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available (potentially including
responses without a Vary header field), the cache will need to choose
one to use. When a selecting header field has a known mechanism for
doing so (e.g., qvalues on Accept and similar request header fields),
that mechanism MAY be used to select preferred responses; of the
remainder, the most recent response (as determined by the Date header
field) is used, as per Section 4.
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.3) request.
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A fresh response is one whose age has not yet exceeded its freshness
lifetime. Conversely, a stale response is one where it has.
A response's freshness lifetime is the length of time between its
generation by the origin server and its expiration time. An explicit
expiration time is the time at which the origin server intends that a
stored response can no longer be used by a cache without further
validation, whereas a heuristic expiration time is assigned by a
cache when no explicit expiration time is available.
A response's age is the time that has passed since it was generated
by, or successfully validated with, the origin server.
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using
either the Expires header field (Section 5.3) or the max-age response
directive (Section 5.2.2.8). Generally, origin servers will assign
future explicit expiration times to responses in the belief that the
representation is not likely to change in a semantically significant
way before the expiration time is reached.
If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will
normally validate a stale cached response before reusing it for
subsequent requests (see Section 4.2.4).
Since origin servers do not always provide explicit expiration times,
caches are also allowed to use a heuristic to determine an expiration
time under certain circumstances (see Section 4.2.2).
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in Section 4.2.1; current_age is
defined in Section 4.2.3.
Clients can send the max-age or min-fresh cache directives in a
request to constrain or relax freshness calculations for the
corresponding response (Section 5.2.1).
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When calculating freshness, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive, a
cache recipient SHOULD match day, week, and time-zone names case-
insensitively.
o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value.
o A cache recipient MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.
o A cache recipient SHOULD consider a date with a zone abbreviation
other than GMT or UTC to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a
resource. See Section 6 for an explanation of the difference between
caches and history mechanisms.
4.2.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of the
following:
o If the cache is shared and the s-maxage response directive
(Section 5.2.2.9) is present, use its value, or
o If the max-age response directive (Section 5.2.2.8) is present,
use its value, or
o If the Expires response header field (Section 5.3) is present, use
its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see
Section 4.2.2.
Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server.
When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age
directives), the directive's value is considered invalid. Caches are
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encouraged to consider responses that have invalid freshness
information to be stale.
4.2.2. Calculating Heuristic Freshness
Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other header field
values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable by default (see
Section 6.1 of [SEMNTCS]), and those responses without explicit
freshness that have been marked as explicitly cacheable (e.g., with a
"public" response directive).
If the response has a Last-Modified header field (Section 2.2 of
[CONDTNL]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD generate a Warning header field with a 113 warn-code (see
Section 5.5.4) in the response if its current_age is more than 24
hours and such a warning is not already present.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those
containing '?'). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send
explicit directives (e.g., Cache-Control: no-cache) if they wish
to preclude caching.
4.2.3. Calculating Age
The Age header field is used to convey an estimated age of the
response message when obtained from a cache. The Age field value is
the cache's estimate of the number of seconds since the response was
generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
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The following data is used for the age calculation:
age_value
The term "age_value" denotes the value of the Age header field
(Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available.
date_value
The term "date_value" denotes the value of the Date header field,
in a form appropriate for arithmetic operations. See
Section 7.1.1.2 of [SEMNTCS] for the definition of the Date header
field, and for requirements regarding responses without it.
now
The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC5905])
or some similar protocol to synchronize its clocks to Coordinated
Universal Time.
request_time
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the
response was received.
A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by
zero.
2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this
value relative to the time the request was initiated, not the
time that the response was received.
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
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These are combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header field
(e.g., because there are no HTTP/1.0 hops in the Via header field),
in which case the corrected_age_value MAY be used as the
corrected_initial_age.
The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
4.2.4. Serving Stale Responses
A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 4.2.
A cache MUST NOT generate a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 5.2.2).
A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a
forward path) or doing so is explicitly allowed (e.g., by the max-
stale request directive; see Section 5.2.1).
A cache SHOULD generate a Warning header field with the 110 warn-code
(see Section 5.5.1) in stale responses. Likewise, a cache SHOULD
generate a 112 warn-code (see Section 5.5.3) in stale responses if
the cache is disconnected.
A cache SHOULD NOT generate a new Warning header field when
forwarding a response that does not have an Age header field, even if
the response is already stale. A cache need not validate a response
that merely became stale in transit.
4.3. Validation
When a cache has one or more stored responses for a requested URI,
but cannot serve any of them (e.g., because they are not fresh, or
one cannot be selected; see Section 4.1), it can use the conditional
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request mechanism [CONDTNL] in the forwarded request to give the next
inbound server an opportunity to select a valid stored response to
use, updating the stored metadata in the process, or to replace the
stored response(s) with a new response. This process is known as
"validating" or "revalidating" the stored response.
4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache
sends one or more precondition header fields containing validator
metadata from its stored response(s), which is then compared by
recipients to determine whether a stored response is equivalent to a
current representation of the resource.
One such validator is the timestamp given in a Last-Modified header
field (Section 2.2 of [CONDTNL]), which can be used in an If-
Modified-Since header field for response validation, or in an If-
Unmodified-Since or If-Range header field for representation
selection (i.e., the client is referring specifically to a previously
obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag header field
(Section 2.3 of [CONDTNL]). One or more entity-tags, indicating one
or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with
the listed entity-tags).
4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is
common for a cache at an intermediary to receive conditional requests
from other (outbound) caches. Likewise, some user agents make use of
conditional requests to limit data transfers to recently modified
representations or to complete the transfer of a partially retrieved
representation.
If a cache receives a request that can be satisfied by reusing one of
its stored 200 (OK) or 206 (Partial Content) responses, the cache
SHOULD evaluate any applicable conditional header field preconditions
received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin
server, found in a request with semantics that cannot be satisfied
with a cached response, or applied to a target resource for which it
has no stored responses; such preconditions are likely intended for
some other (inbound) server.
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The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as
defined in Section 6 of [CONDTNL]. The If-Match and If-Unmodified-
Since conditional header fields are not applicable to a cache.
A request containing an If-None-Match header field (Section 3.2 of
[CONDTNL]) indicates that the client wants to validate one or more of
its own stored responses in comparison to whichever stored response
is selected by the cache. If the field-value is "*", or if the
field-value is a list of entity-tags and at least one of them matches
the entity-tag of the selected stored response, a cache recipient
SHOULD generate a 304 (Not Modified) response (using the metadata of
the selected stored response) instead of sending that stored
response.
When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the
forwarded request. If a stored response contains only partial
content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4).
If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [CONDTNL])
indicates that the client wants to validate one or more of its own
stored responses by modification date. A cache recipient SHOULD
generate a 304 (Not Modified) response (using the metadata of the
selected stored response) if one of the following cases is true: 1)
the selected stored response has a Last-Modified field-value that is
earlier than or equal to the conditional timestamp; 2) no Last-
Modified field is present in the selected stored response, but it has
a Date field-value that is earlier than or equal to the conditional
timestamp; or, 3) neither Last-Modified nor Date is present in the
selected stored response, but the cache recorded it as having been
received at a time earlier than or equal to the conditional
timestamp.
A cache that implements partial responses to range requests, as
defined in [RANGERQ], also needs to evaluate a received If-Range
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header field (Section 3.2 of [RANGERQ]) with respect to its selected
stored response.
4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request is dependent
upon its status code:
o A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.3.4.
o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request
is suitable. Instead, the cache MUST use the full response to
satisfy the request and MAY replace the stored response(s).
o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed
to respond. In the latter case, the cache MAY send a previously
stored response (see Section 4.2.4).
4.3.4. Freshening Stored Responses upon Validation
When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new
information provided in the 304 response.
The stored response to update is identified by using the first match
(if any) of the following:
o If the new response contains a strong validator (see Section 2.1
of [CONDTNL]), then that strong validator identifies the selected
representation for update. All of the stored responses with the
same strong validator are selected. If none of the stored
responses contain the same strong validator, then the cache MUST
NOT use the new response to update any stored responses.
o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected for update.
o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since
request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored
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response also lacks a validator, then that stored response is
selected for update.
If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header
fields in the stored response.
4.3.5. Freshening Responses via HEAD
A response to the HEAD method is identical to what an equivalent
request made with a GET would have been, except it lacks a body.
This property of HEAD responses can be used to invalidate or update a
cached GET response if the more efficient conditional GET request
mechanism is not available (due to no validators being present in the
stored response) or if transmission of the representation body is not
desired even if it has changed.
When a cache makes an inbound HEAD request for a given request target
and receives a 200 (OK) response, the cache SHOULD update or
invalidate each of its stored GET responses that could have been
selected for that request (see Section 4.1).
For each of the stored responses that could have been selected, if
the stored response and HEAD response have matching values for any
received validator fields (ETag and Last-Modified) and, if the HEAD
response has a Content-Length header field, the value of Content-
Length matches that of the stored response, the cache SHOULD update
the stored response as described below; otherwise, the cache SHOULD
consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a
HEAD response, the cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
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o use other header fields provided in the HEAD response to replace
all instances of the corresponding header fields in the stored
response and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control
header field.
4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [SEMNTCS]) such as
PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents
up to date.
A cache MUST invalidate the effective Request URI (Section 5.5 of
[MESSGNG]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error status code is
received in response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5
of [MESSGNG]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of
[MESSGNG]) when it receives a non-error response to a request with a
method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will
either remove all stored responses related to the effective request
URI or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent
request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not.
5. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
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The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in
Section 4.2.3.
Age = delta-seconds
The Age field-value is a non-negative integer, representing time in
seconds (see Section 1.2.1).
The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an
HTTP/1.0 cache that does not implement Age.
5.2. Cache-Control
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does
not imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control directives
defined in this section. See Section 5.2.3 for information about how
Cache-Control directives defined elsewhere are handled.
Note: Some HTTP/1.0 caches might not implement Cache-Control.
A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their
significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is
not possible to target a directive to a specific cache.
Cache directives are identified by a token, to be compared case-
insensitively, and have an optional argument, that can use both token
and quoted-string syntax. For the directives defined below that
define arguments, recipients ought to accept both forms, even if one
is documented to be preferred. For any directive not defined by this
specification, a recipient MUST accept both forms.
Cache-Control = 1#cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ]
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For the cache directives defined below, no argument is defined (nor
allowed) unless stated otherwise.
5.2.1. Request Cache-Control Directives5.2.1.1. max-age
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale
response.
This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.1.2. max-stale
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its freshness
lifetime. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a stale
response of any age.
This directive uses the token form of the argument syntax: e.g.,
'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
the quoted-string form.
5.2.1.3. min-fresh
Argument syntax:
delta-seconds (see Section 1.2.1)
The "min-fresh" request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less than
its current age plus the specified time in seconds. That is, the
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client wants a response that will still be fresh for at least the
specified number of seconds.
This directive uses the token form of the argument syntax: e.g.,
'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
the quoted-string form.
5.2.1.4. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
5.2.1.5. no-store
The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.
5.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [MESSGNG].
5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches.
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The "must-revalidate" response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate
a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result
in incorrect operation, such as a silently unexecuted financial
transaction.
5.2.2.2. no-cache
Argument syntax:
#field-name
The "no-cache" response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without contacting
it, even by caches that have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header
fields in the response that have the field-name(s) listed MUST NOT be
sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response.
The field-names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists).
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Note: Although it has been back-ported to many implementations, some
HTTP/1.0 caches will not recognize or obey this directive. Also, no-
cache response directives with field-names are often handled by
caches as if an unqualified no-cache directive was received; i.e.,
the special handling for the qualified form is not widely
implemented.
5.2.2.3. no-store
The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
5.2.2.4. no-transform
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [MESSGNG].
5.2.2.5. public
The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or
cacheable only within a private cache. (See Section 3.2 for
additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable by default; see
Section 4.2.2.)
5.2.2.6. private
Argument syntax:
#field-name
The "private" response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared
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cache. A private cache MAY store the response and reuse it for later
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the
remainder of the response message.
The field-names given are not limited to the set of header fields
defined by this specification. Field names are case-insensitive.
This directive uses the quoted-string form of the argument syntax. A
sender SHOULD NOT generate the token form (even if quoting appears
not to be needed for single-entry lists).
Note: This usage of the word "private" only controls where the
response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are
often handled by caches as if an unqualified private directive was
received; i.e., the special handling for the qualified form is not
widely implemented.
5.2.2.7. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to
private caches.
5.2.2.8. max-age
Argument syntax:
delta-seconds (see Section 1.2.1)
The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number
of seconds.
This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.2.9. s-maxage
Argument syntax:
delta-seconds (see Section 1.2.1)
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The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
This directive uses the token form of the argument syntax: e.g.,
's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the
quoted-string form.
5.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value. A cache
MUST ignore unrecognized cache directives.
Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other
directives.
Behavioral extensions are designed to work by acting as modifiers to
the existing base of cache directives. Both the new directive and
the old directive are supplied, such that applications that do not
understand the new directive will default to the behavior specified
by the old directive, and those that understand the new directive
will recognize it as modifying the requirements associated with the
old directive. In this way, extensions to the existing cache-control
directives can be made without breaking deployed caches.
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive: in
addition to private caches, any cache that is shared only by members
of the named community is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI"
A cache that recognizes such a community cache-extension could
broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache-extension would ignore it and
adhere to the private directive.
5.3. Expires
The "Expires" header field gives the date/time after which the
response is considered stale. See Section 4.2 for further discussion
of the freshness model.
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The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The Expires value is an HTTP-date timestamp, as defined in
Section 7.1.1.1 of [SEMNTCS].
Expires = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already
expired").
If a response includes a Cache-Control field with the max-age
directive (Section 5.2.2.8), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive
(Section 5.2.2.9), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field.
An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or
user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more
than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner
than that.
5.4. Pragma
The "Pragma" header field allows backwards compatibility with
HTTP/1.0 caches, so that clients can specify a "no-cache" request
that they will understand (as Cache-Control was not defined until
HTTP/1.1). When the Cache-Control header field is also present and
understood in a request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This
specification deprecates such extensions to improve interoperability.
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Pragma = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
When the Cache-Control header field is not present in a request,
caches MUST consider the no-cache request pragma-directive as having
the same effect as if "Cache-Control: no-cache" were present (see
Section 5.2.1).
When sending a no-cache request, a client ought to include both the
pragma and cache-control directives, unless Cache-Control: no-cache
is purposefully omitted to target other Cache-Control response
directives at HTTP/1.1 caches. For example:
GET / HTTP/1.1
Host: www.example.com
Cache-Control: max-age=30
Pragma: no-cache
will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response.
Note: Because the meaning of "Pragma: no-cache" in responses is
not specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
5.5. Warning
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be
reflected in the status code. This information is typically used to
warn about possible incorrectness introduced by caching operations or
transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures.
Warning header fields can in general be applied to any message,
however some warn-codes are specific to caches and can only be
applied to response messages.
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Warning = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[ SP warn-date ]
warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding
; the Warning header field, for use in debugging
; a single "-" is recommended when agent unknown
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be generated in a response (either by the
origin server or by a cache), including multiple warnings with the
same warn-code number that only differ in warn-text.
A user agent that receives one or more Warning header fields SHOULD
inform the user of as many of them as possible, in the order that
they appear in the response. Senders that generate multiple Warning
header fields are encouraged to order them with this user agent
behavior in mind. A sender that generates new Warning header fields
MUST append them after any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored
response after validation:
o 1xx warn-codes describe the freshness or validation status of the
response, and so they MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation.
o 2xx warn-codes describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of
the representation) and they MUST NOT be deleted by a cache after
validation, unless a full response is sent, in which case they
MUST be.
If a sender generates one or more 1xx warn-codes in a message to be
sent to a recipient known to implement only HTTP/1.0, the sender MUST
include in each corresponding warning-value a warn-date that matches
the Date header field in the message. For example:
HTTP/1.1 200 OK
Date: Sat, 25 Aug 2012 23:34:45 GMT
Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
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Warnings have accompanying warn-text that describes the error, e.g.,
for logging. It is advisory only, and its content does not affect
interpretation of the warn-code.
If a recipient that uses, evaluates, or displays Warning header
fields receives a warn-date that is different from the Date value in
the same message, the recipient MUST exclude the warning-value
containing that warn-date before storing, forwarding, or using the
message. This allows recipients to exclude warning-values that were
improperly retained after a cache validation. If all of the warning-
values are excluded, the recipient MUST exclude the Warning header
field as well.
The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning.
The procedure for defining additional warn codes is described in
Section 7.2.1.
5.5.1. Warning: 110 - "Response is Stale"
A cache SHOULD generate this whenever the sent response is stale.
5.5.2. Warning: 111 - "Revalidation Failed"
A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
5.5.3. Warning: 112 - "Disconnected Operation"
A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time.
5.5.4. Warning: 113 - "Heuristic Expiration"
A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than
24 hours.
5.5.5. Warning: 199 - "Miscellaneous Warning"
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the
user.
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This Warning code MUST be added by a proxy if it applies any
transformation to the representation, such as changing the content-
coding, media-type, or modifying the representation data, unless this
Warning code already appears in the response.
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT
take any automated action.
6. History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
retrieved earlier in a session.
The freshness model (Section 4.2) does not necessarily apply to
history mechanisms. That is, a history mechanism can display a
previous representation even if it has expired.
This does not prohibit the history mechanism from telling the user
that a view might be stale or from honoring cache directives (e.g.,
Cache-Control: no-store).
7. IANA Considerations7.1. Cache Directive Registry
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created
and is now maintained at <http://www.iana.org/assignments/http-cache-directives>.
7.1.1. Procedure
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
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The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
8. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in HTTP messaging
[MESSGNG] and semantics [SEMNTCS].
Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive
information.
In particular, various attacks might be amplified by being stored in
a shared cache; such "cache poisoning" attacks use the cache to
distribute a malicious payload to many clients, and are especially
effective when an attacker can use implementation flaws, elevated
privileges, or other techniques to insert such a response into a
cache. One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see
Section 3.3.3 of [MESSGNG] for the relevant requirements.
Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing
it to unauthorized parties.
Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one
browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more
quickly, thanks to the cache.
Note that the Set-Cookie response header field [RFC6265] does not
inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields.
Fielding, et al. Expires September 6, 2018 [Page 35]